High performance infrared detector packages generally employ some form of coldshielding to limit radiation on the detector element in order to maintain an optimum performance available within the given characteristics of the optical system, object and detector. Typically such coldshields are mounted upon the cold focal plane to establish an efficient definition of the desired ray cone. However, this technique increases both the physical and the thermal mass at the cryogenic focal plane.
Warmshielding, employing an uncooled reflector, has also been employed in cryogenic infrared detector packages in order to reduce the physical size of the coldshield. Warmshields block radiation from outside the detector's field of view, but may still increase heatload and thermal mass on the cooler due to a requirement that they image the detector upon a cold target, typically the focal plane surrounding the detector and/or the coldshield.
A particular problem associated with cryogenic packages that employ thermoelectrically cooled detectors is that a small increase in heatload at the coldest stage or stages significantly increases the cold-end temperature and/or significantly increases the power dissipated by the cooler. For example, one known type of thermoelectric cooler, operated with a heatsink temperature of 50.degree. C. that attains a detector stage temperature of -88.degree. C. (185 K) experiences a temperature increase under heatload of approximately 0.3.degree. C/mW Thus, an additional cold end heatload of 10 mW increases the detector temperature from 185 K to approximately 188 K.
It is therefore one object of the invention to provide a radiation shield for a cryogenically cooled radiation detector that does not increase either the physical or the thermal mass at the detector plane.
It is another object of the invention to provide a radiation shield for a cryogenically cooled radiation detector that significantly reduces radiative transfer into the coldest stages of a thermoelectric cooler, thereby lowering the temperature of the detector and/or reducing cooler power consumption.
It is another object of the invention to provide a radiation shield for a cryogenically cooled radiation detector that improves the overall detector assembly ruggedness and producibility by mounting the shield to structure other than a thermoelectric cooler.
It is still another object of the invention to provide a radiation shield for a cryogenically cooled radiation detector that provides a physically large shield cantilevered forward of the detector and that permits an optimum positioning of the cold aperture stop relative to the detector.
It is a further object of the invention to place energy absorbing materials within the radiation shield at locations where temperatures are sufficiently low that unwanted radiative self-emission is not introduced, and where optical reflections are intercepted, so that extraneous energy is eliminated and not coupled through an integrating-sphere effect into the coldest, or detector, stage. The energy so inhibited from this unwanted transfer includes energy that enters the coldstop obliquely and is not directly incident upon the coldest stage, as well as energy that has been reflected by the coldest stage.